The present invention generally relates to a communication system, and particularly to a mapping method of code word in radio communication apparatus used in a radio communication system.
FIG. 1 is a block diagram of the constructions of conventional radio communication apparatus and radio communication system. The radio communication system is formed of a transmission-side radio communication apparatus 101 and a receiving-side radio communication apparatus 102 between which messages and data are transmitted through a radio propagation path 103. Although in FIG. 1 and all the following drawings one channel (for example, downlink) is given as an example, the same is true of uplink.
In the transmission-side radio communication apparatus 101, the data to be transmitted is first encoded by a coder 107 within a transmitter 105. In radio communication, convolutional coding and turbo coding are often used as coding methods. The coded words, or code words are supplied to and modulated by a QAM (Quadrature Amplitude Modulation) modulator 106. The modulated baseband signal after the modulation is converted to a radio frequency band by an RF unit 108, and then transmitted. On the receiving-side radio communication apparatus 102, the radio signal received from the radio propagation path 103 via an antenna 110 is converted to the baseband signal by an RF unit 114. The baseband signal is first demodulated by a QAM demodulator 112 of a receiver 111, and then decoded by a decoder 113.
It is here assumed that the coder in the transmission-side apparatus makes turbo coding of a coding rate of R=⅓ and constraint length K=4.
FIG. 2 shows an example of the construction of a turbo encoder that makes turbo coding of a coding rate of R=⅓ and constraint length K=4. This coding is one of the widely used coding methods as stipulated by 3GPP (3rd Generation Partnership Project) specification TS25.212 for the third generation mobile communication. This turbo encoder is formed of two convolution coders and an interleaver provided within the turbo encoder. When a one-bit input 201 is applied to that encoder, three bits 203, 204, 205 are produced from the output. Particularly, it is characterized that output bit 203 is produced as the input bit itself. For the sake of convenience, the output bit produced as the input bit is called x-bit, the convolution-coded output called y-bit, and the output bit convolution-coded after interleaving within the turbo encoder called y′-bit.
The QAM modulator 106 in the transmission-side apparatus 101 makes QAM that has been investigated from old times in order to increase the transmission efficiency.
The error rate in the conventional demodulation and modulation will be mentioned below.
FIG. 3 shows the arrangement of signal points for multi-level modulation of 64 QAM in the transmission-side QAM modulator 106. In 64 QAM, 6 bits per symbol can be transmitted. As shown in FIG. 3 (by 301), 6 bits {S5, S4, S3, S2, S1, S0} of each symbol are grouped into three in-phase component bits {S2, S1, S0} and three quadrature component bits {S5, S4, S3}, and the symbols are subjected to gray coding in order that the adjacent symbols have one-bit difference, arranged at the signal points as shown in FIG. 3, and transmitted.
The QAM demodulator 112 of the receiver 111 in the receiving-side apparatus 102 makes demodulation of the received signals according to their receiving points. FIG. 4 shows a demodulating method in which hard decision is made of whether the transmitted signal is “0” or “1” depending on the position of received signal. The S5-bit assigned to the most significant bit of, for example, quadrature component {S5, S4, S3} is decided “0” if the quadrature component of the received signal is positive, or “1” if it is negative as shown in FIG. 4 (at 405). If, for example, “0” is transmitted as information, and noise is added to the received signal, then error occurs when the value of the quadrature component becomes negative across the decision threshold, received signal amplitude (=0). Therefore, the more distant the received signal from the decision threshold, the more unlikely it is that error occurs, that is, the resistance to noise is strong.
From the observation of demodulation (405) of S5-bit and demodulation (402) of S2-bit, it will be understood that the threshold for decision is the same received signal level (amplitude)=0 except that the axes to be noted are different. In other words, the resistance (communication quality) of S5-bit to noise can be considered to be the same as that of S2-bit.
In addition, from the demodulation (405) of S5-bit and demodulation (403) of S3-bit, it will be seen that since the threshold for decision of “0”, “1” often occurs for S3-bit, the resistance (communication quality) of S3-bit to noise is weak.
These values will be quantitatively evaluated. FIG. 5 shows the error rate characteristic of each of 6 bits that can be transmitted per symbol of 64 QAM. From the comparison of necessary S/N (ratio of signal power to noise power) for error rate =10−1 in FIG. 5, it will be seen that the medium bit {S4, S1) is about 12 dB necessary which is 6 dB larger than the most significant bit {S5, S2} of 6 dB that has strong resistance to error, and which is 3 dB less than the least significant bit {S3, S0} of about 15 dB that has weak resistance to error.
Although the above description was made for hard decision, the demodulated data from the QAM demodulator 112 has a likelihood (probability) provided by soft decision because data is decoded by the decoder 113 after demodulation by the QAM demodulator 112 of receiver 111 in the receiving-side apparatus 102.
This radio communication system considers the mapping of turbo-encoded code word bits to bits of the 64 QAM symbol. As illustrated in FIG. 6, when a code word sequence 601 is mapped into bits (602) of QAM, the x-bit (603) of the code word is produced at every third bit because of coding rate R=⅓. In addition, this x-bit is every time mapped into S5-bit or S2-bit that is the strongest to error in 64 QAM but never mapped into other bits (701). FIG. 7 shows the code word mapping of a fixed mapping method. FIG. 8 shows an error rate characteristic of the fixed mapping method. This mapping method (here called fixed mapping) can be used for y-bit or y′-bit of other code words. For example, y-bits are all assigned to the medium bit {S4, S1} of 64 QAM (702), and y′-bits every time to the S2-bit, SO-bit of 64 QAM that is the lowest resistance to error (703). Therefore, the conventional radio communication apparatus constructed by the combination of 64 QAM and turbo coding of cording rate R=⅓ cannot attain a steep error characteristic irrespective of using the turbo coding shown in FIG. 8.
This error rate deterioration is caused by the fixed mapping shown in FIG. 7. The method for overcoming this problem is to interleave. The interleaving is the technique for previously changing the order of bits of a bit sequence to be transmitted in order to strengthen the resistance to burst error that occurs particularly in radio communication.
FIG. 9 shows the situation in which bits of each turbo code word are mapped into bits of 64 QAM at the time of interleaving. That is, each code word is equally mapped into bits from the most significant bit to the least significant bit of QAM (901, 902, 903).
FIG. 10 is a block diagram showing the construction of a radio communication system with an interleaver added. A transmitter 1003 of a transmission-side radio communication apparatus 1001 makes turbo coding by the coder 107, makes interleaving (1004), and then makes QAM modulation (106) by mapping into QAM bits. A receiver 1005 of a receiving-side apparatus 1002 makes QAM demodulation (112), deinterleaving (1006) for restoring the interleaved sequence to the original one, and then decoding of turbo code by the decoder 113. FIG. 11 is a graph of error rate characteristic with this interleaver added. The interleaving process can cause the error rate characteristic to have steepness peculiar to the turbo code in the high S/N region. In the low S/N region, however, the interleaving deteriorates the error rate characteristic.
Thus, after detailed examination of the error rate characteristics in the prior art, the following problems have been found.
In the conventional radio communication system, since the fixed mapping method with no interleaver is used in which each code word bits are every time assigned to the same bits of QAM, the communication quality of each code word is changed, thus causing the error rate characteristic deterioration shown in FIG. 8.
Also, in the conventional system, when the bits of code words are equally assigned to QAM bits by adding an interleaver, the error rate can be improved in the high S/N region so that a steep error rate characteristic peculiar to turbo code can be obtained, but contrarily in the low S/N region the error rate characteristic is deteriorated by the interleaving as shown in FIG. 11.
In addition, the conventional system has the drawback that the code word mapping method cannot be changed according to the situation of propagation path and S/N.
Moreover, in the conventional system, the code word mapping method does not consider the level of importance of code word for decoding and the resistance of QAM bits to error, and thus satisfactory error rate characteristics cannot be achieved in all S/N regions.